1. Field of the Invention
The present invention generally relates to an apparatus and a method for enhancing speech intelligibility and outputting speech with the enhanced intelligibility in a mobile terminal. More particularly, the present invention relates to an apparatus and a method for enhancing speech intelligibility and outputting speech with the enhanced intelligibility by emphasizing a speech signal in a mobile terminal.
2. Description of the Related Art
Mobile terminals including hand-held phones can be used in environments with ambient noise like an airport or a station platform. Due to the ambient noise in the listener environment, the mobile terminals may provide very unintelligible speech to listeners.
Conventionally, the mobile terminals use a clipping circuit or an equalizer circuit to control output sound volume, or adopt a formant method in order to minimize noise corruption to speech intelligibility in a real environment.
Clipping is the simplest technique for enhancing speech intelligibility. Specific samples are clipped in an input signal and the entire signal is amplified. By use of an equalizer circuit, the mobile terminals can enhance speech intelligibility by converting an input signal to a high frequency range (2 KHz or higher). The volume control scheme increases the output sound volume in the presence of ambient noise and provides the increased volume to the listener.
However, the above three conventional methods amplify both a noise signal and a speech signal by amplifying an input signal. As a consequence, speech intelligibility drops.
Besides, speech intelligibility can be enhanced using peaks called formants in the frequency spectrum of a speech signal. The frequency spectrum of a speech signal involves three or fewer formants. In the case of three formants, these are called first, second and third formants in the order of low-to-high frequencies. This formant method enhances speech intelligibility by emphasizing high-order (the second and third) formants based on the property that amplitude (power) decreases in higher frequency in the speech spectrum. While the formant method can enhance speech intelligibility if only speech spectrum exists in a frequency band, it may decrease the speech intelligibility because components other than the formants are also emphasized in the case where the noise spectrum and the speech spectrum co-exist in the frequency band.
Accordingly, there exists a need for a new technique for enhancing speech intelligibility for a mobile terminal in a real noisy environment.